An acoustic wave device using a piezoelectric thin film resonator is used as a filter and a duplexer of a wireless device such as a mobile phone. The piezoelectric thin film resonator has a structure a lower electrode and an upper electrode sandwich a piezoelectric film and face with each other.
As a wireless system rapidly spreads, many frequency bands are being used. As a result, sharpening of skirt characteristic of a filter or a duplexer is requested. There is a method of enlarging a Q value of a piezoelectric thin film resonator as one method of sharpening the skirt characteristic. Acoustic wave energy may be leaked to outside from a resonance region, as a factor of degradation of the Q value of a piezoelectric thin film resonator.
Japanese Patent Application Publication No. 2006-109472 (hereinafter referred to as Document 1) discloses that the Q value is improved by providing a ring band on a surface of a lower electrode or an upper electrode.